Iran had been silent about the Jamal Khashoggi case until Monday.
Saudi journalist Jamal Khashoggi's "heinous murder" would have been unthinkable "without US backing," Iranian President Hassan Rouhani said on Wednesday.
"I don't think that any country would dare do such a thing without US backing," Rouhani said in remarks to cabinet broadcast on state television.
Rouhani said that before Khashoggi's murder "it would have been unthinkable that in this day and age we would witness such an organised felony.
"It is extremely significant that an institution planned such a heinous murder.
"The tribal group that is ruling that nation (Saudi Arabia) has a security margin. That security margin is that it relies on US backing. It is this superpower that is backing them."
Khashoggi, a government critic who was living in self-imposed exile in the United States, was murdered inside the Saudi consulate in Istanbul on October 2 as he organised the paperwork for his marriage to his Turkish fiancee.
The case has triggered an international outcry against Saudi Arabia. US President Donald Trump on Tuesday ridiculed Riyadh's response as "one of the worst cover-ups" in history.
Iran had been silent about the Khashoggi case until Monday when its judiciary chief slammed Saudi Arabia over his death.
"This heinous murder further revealed the nature of Saudis, their kingdom and that young man who is seeking fame and murdering innocent people," Sadegh Amoli Larijani was quoted as saying by the judiciary's news agency Mizan Online, apparently referring to Saudi Crown Prince Mohammad bin Salman.
'Child-killing regime'
Iran has closely followed the backlash faced by its regional rival Saudi Arabia over the Khashoggi murder.
Iranian Foreign Minister Mohammad Javad Zarif on Wednesday lashed out at the US Treasury for sanctioning two Iranian Revolutionary Guard officers who allegedly provided training, financial and logistical support to Afghanistan's Taliban.
"To deflect from headlines on Saudi brutality in Istanbul and across Yemen, US Treasury -- while in Saudi Arabia, no less -- sanctions Iran for 'supporting' anti-Iran Taliban," he tweeted.
The sanctions were announced by the Riyadh-based Terrorist Financing Targeting Center on Tuesday.
Separately Saudi Arabia and Bahrain also on Tuesday designated Iran's Islamic Revolutionary Guard Corps and three of its officers, including Quds Force commander Major General Qasem Soleimani, as terrorist sponsors.
"The House of Saud is seeking to divert international and regional public opinion away from the murder of Jamal Khashoggi," Brigadier General Esmail Kowsari of the Guards said, quoted by Mehr news agency.
"Saudi Arabia is stuck in a quagmire they can't get out of," Kowsari said, adding that the designation was an attempt to utilise the tactic of attack being the best form of defence.
"The murder of this journalist is one of the thousands of villainies committed by this child-killing regime, but they want to revolve the world's attention away from themselves onto Iran," said Kowsari.